Stereophonic conference system



Dec. 14, 1943. M L 2,336,880

STEREOPHONIC CONFERENCE SYSTEM Filed May 28, 1942 2 Sheets-Sheet? 41,4575? max 32 East INVENTOR 2 i llflfitchel ATi'ORNEY Patented Dec. 14, 1943 2,336,880 STEREOPHONIC CONFERENCE SYSTEM Doren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation or New York Application May 28, 1942, Serial No. 444,826

8 Claims.

This invention relates to a two-way speech transmission system characterized by the use at each terminal of two microphonic pick-up devices, each connected to its own transmission circuit, for the transmission of speech therefrom, and also having at each terminal two speech-producing devices, each connected to its own receiving circuit, whereby a stereophonic characteristic may be produced in the transmitted and received speech.

Two-way speech transmission system having separate transmitting and' receiving circuits at each terminal of such a system have been employed in the so-called conference systems by which a group of persons at one location may maintain a conference with a group at a remote location by telephone circuits extending between such locations. Such a system is shown in the patent to Mitchell, 2,267,622, dated December. 23, 1941. The system therein disclosed employs a single transmitting circuit and a single receiving circuit, the transmitting circuit having a transmitter, and the receiving circuit a loud-speaker at each terminal of the system. It has been found that by employing two transmitting circuits and two receiving circuits, each transmitting circuit having a transmitter connected therewith, and each receiving circuit having a loud-speaker connected therewith, it is possible to produce in the listeners a sense of location of the speaker, with respect to the transmitters, which adds greatly to the naturalness of the speech that is received at the distant end of the transmitting circuits. Such a system has an additional advantage in that it increases the chances of understanding speech under noisy conditions that may at times exist in a conference where this invention may be employed. The invention likewise may be employed in connection with television systems, being adapted to give a stereo phonic effect that would be desirable in connection with television.

These and other objects of this invention and their manner of attainment will be apparent from the following description when read in connection with the attached drawings of which Fig. 1 shows schematically a form of the embodiment of the invention, and Fig. 2 shows in detail the apparatus and the circuit arrangements required for carrying out the invention shown broadly in Fig. 1.

In Fig. 1, M1 and M2 represent microphones at the west station of the system, and IS: and LS4 represent loud-speakers at the same station. Each of the microphones is connected to a transmitting circuit that extends from the west station to the east station, such circuits comprising a pair of wires represented by a single line in Fig. 1. station terminates in loud-speakers LS1 and LS: at the east station, and at that same station the microphones M3 and M4 are connected to a transmitting circuit which becomes the receiving circuit at the west station. The voice waves from a speaker at the west station, falling upon the microphones M1 and M2, will produce in them an effect which, in the point of time, will depend upon the relative locations of the speaker and each microphone. The speech currents resulting therefrom will pass through the volume limiters VII]. and Vile, and if the suppressors SP1 and SP2 are closed, as indicated by the touching arrowheads of the figure, the speech currents will pass over the circuits to the east station. The volume of speech transmitted will depend upon the setting of the volume limiters, and that in turn will be controlled by the output of the detector D1, the input of which is connected with both transmitting circuits through the amplifiers A1 and A2. In that way the volume in both paths may be held below the overload point in the usual manner but in such a way as to keep the loss in both paths always the same, thus maintaining the proper relationship for stereophonic transmission.

To allow break-in when the talker raises his voice, a portion of the speech energy is diverted into the circuit containing the amplifiers A3 and. A4, the output of which is connected to the detector D2. The detected current serves to operate the break relay BK12, which disables the suppressors SP1 and SP2, and thus allows the west talker to break in. The currents transmitted from the west station will pass through the expanders EX1 and EX: to the loud-speakers LS1 and LS2, the connection including amplifiers A5 and As. A portion of the received energy is diverted into the control circuits of the expanders passing through the amplifiers A7 and A8, which are connected to the detector D3, which in turn controls the expanders. Some of this diverted current also is rectified by the detector D4, and operates the suppressors SP3 and SP4 in the circuit that transmits from the east station to the west station. The operation of those suppressors opens the east to west transmitting circuit and prevents singing that may result from sound waves from the loud-speakers at the east station falling upon the microphones thereat.

If the speaker at the east station desires to get control of the system, he may do so by raising The transmitting circuit from the west the volume level of his voice and thereby operate the break relay BK, That result is effected by the current that is diverted into the circuit of the amplifiers A9 and Am, the output of which, when rectified by the detector D5, serves to operate break relay BK34. The operation of that break relay restores the contacts of the suppressors SP3 and SPlso that speech currents from the microphones Ma and M4 will pass over the westbound circuits to the west station. If the volume level of speech received at the west station is sufiiciently high, it will serve to operate the guard relay G12. That is accomplished by the current that passes through the amplifiers A11 and A12. which is rectified by the detector D5 and impressed upon the guard relay G12. The operation of that relay deenergizes the break relay BKiz, thus preventing its operation on strong echo. If BKia were allowed to operate on such strong echo, it would disable SPr and SP2, thus allowing echo to get back to the east station and perhaps even allowing singing.

The speech currents being transmitted from the east to west for the actuation of the loud-speak,- ersLSs and LS; will pass through the expanders Em and EX4, which are controlled'by the rectified output of the detector D7. All of the expanders of this system are preferably of a type having a fixed loss normally present in them, which loss is adapted to be taken out by the operation of the control devices. The removal of the losses from each pair of expanders is effected simultaneously and equally by the rectified current of the associated detector, which is connected to both circuits through the associated amplifiers.

' It will thus be seen that the speech currents from the west station will pass over the transmitting circuits to the east station, and the speech will be reproduced thereat by the loud-speakers LS1 and LS2 provided, of course, that the suppressors SP1 and SP2 are closed. Those speech currents from west to cast will likewise normally operate the suppressors SP3. and SP4 at the east station thus preventing echoes and singing. If, however, the east speaker desires to get control of the circuit he may, by raising the volume of his speech, operate the break relay BK34 which disables suppressors SP3 and SP4 thus permitting transmission from east to west. He cannot break through if west were also talkingloud since in this case the guard relay Gcc would have also been operated. In practice, however, it has been found that talkers seldom speak loudenough continuously to hold the guard relays operated for long periods at a time. Those transmitted currents will then operate the suppressors SP1 and SP2, thereby breaking transmission from west to east.

Fig. 2 shows in detail the circuit arrangements for accomplishing the result set forth broadly above. The microphones M1 and M2 are connected through the volume limiters VL1 and Vie to the circuits l and l to which circuits are connected the amplifiers A1 and A2, which apply their output currents to the detector D1, which controls the volume limiters. The currents in circuits 5 and i, when amplified by the amplifiers 2 and 2', are applied to the transformers 3 and 3', the secondary winding of each of which is divided as indicated by 5 and ii. The voltage across the winding t is applied to a circuit that may be traced from ground at it through back contact of relay it, through winding it, resistances ii, upper contact of relay it, winding e,

aeeaeso resistance 9, to ground at 8, which circuit is shunted in part by resistance l2. The resistance Iii is normally shunted by the path from the upper contact of relay I6, and the circuit as thus arranged permits the maximum transmission therethrough to the line I by which speech is transmitted to the loud-speaker LSr at the east end of the circuit. In like manner the speech currents in circuit i are transmitted to the line i for transmission thereover to the loud-speaker LS2, the apparatus between the transformers 3' and 6' being similar to that between transformers 3 and 6 of the upper transmission circuit.

Winding 5 is connected to the input of the amplifier As, the output of which is connected to the rectifier H. In similar fashion winding 5 is connected to the amplifier A4, the output of which is similarly connected to the said rectifier. The current rectified by ll controls the master break relay 22, which in turn controls the break relay 23. The break relay in turn controls the suppressor relays it and it. The master guard relay 263 is controlled through the rectifier 6?, which is connected through a network to the transformer st to which is applied a voltage that represents the combined efiect oi the currents in the receiving paths 35 and 35 The circuits just traced, including the suppressor circuits between the transformers 3 and f5 and 3' and 5', respectively, are in general similar to those shown in the patent to Mitchell, 2,267,622, to which reference is made for an understanding of the characteristics of certain details which are fully described therein.

Each receiving circuit has therein apparatus (expander) for gradually cutting out a loss so as to increase the output of such circuit. That is represented by the apparatus between the circuits 35 and it and 35 and 36, and also by the apparatus that is connected to the transformer 53. The received speech waves pass from the circuit 35 through the pad 38 to the loss circuit it, and thence through the amplifier 36 to the outgoing circuit at, which is connected through the amplifier A1: to the loud-speaker LS3. In

order to remove the loss normally present in circuit til, a portion of the speech waves is diverted to the circuit 52, which is connected to both incoming circuits 35 and. 35' and is impressed by transformer 53 upon the amplifier tube 55. The

upon the tube 59. The space current or that tube flows from the battery 59 through the winding of the transformer 39', the varistors 6i and 62' in parallel to the mid-point of the primary winding of transformer 53, thence to the midpoint of the secondary winding of transformer. 39, thence through the varistors iii and 52 in parallel to the mid-point of the primary winding of transformer 3, thence by conductor 5b to the plate of the tube 59. An increase of current flow through this circuit decreases the loss in the circuit it, and also til, by lowering the impedance of the varistors ll and t2, and also fit and 52. The, effect of the incoming speech in the branch 52 is to operate the controlled devices that correspond to the expanders E& and EXi of Fig.1, and thereby to remove loss from the expander circuits and thus give an increased volume of speech in the circuits 48 and 48'.

The manner in which the aforedescribed apparatus operates in order to attain the objects of this invention is as follows: With the relays l5, l6 and IS in their normal unoperated condition the speech currents originating in themicrophones M1 and M2 will be transmitted through the suppressor circuits to the lines 1 and 1, and will in turn be transmitted to the loudspeakers at the distant ends of those lines. A portion of such currents will be amplified by A: and A4, and the current resulting from the combination will berectified' by 11 and will operate relay 22 if the talker were speaking loudly. The opening of the right-hand contact of that relay will cause the master suppressor relay I5 to fall back even though it was already operated by the received currents in the circuit 15. Upon the closing of the left-hand contact of relay 22, relay 23 will be energized, and the positive battery associated with relay 23 will immediately charge up condenser 28 if it were partially discharged. Thus relays l8 and i6 will immediately release without having to wait for the normal hangover time. With the latter relays unoperated the shunts around the resistances II] and I are maintained, thereby maintaining through transmission conditions into the lines 1 and I. When the west talker speaks in a normal manner, he will only occasionally operate relays 22 and 23, however.

When the person or persons at the east terminus of the system speak, the resultant speech currents are diverted in part from the lines 35 and 35, and when amplified by A11 and A1: are impressed by the circuits 52 and 15 across the transformer I6. The resultant voltage, when amplified by 36, is impressed across the transformer 31, the secondary winding of which is split.

Let us assume for the moment that no speech of sufficient magnitude is coming from west to operate relays 22 or 23. In this case the voltage in the upperhalf of transformer 31 will be rectified in rectifier 68 and operate the master suppressor relay l5. Operation of this relay immediately disables the transmitting paths by removing ground from transformers i3 and I3 and also initiates operation of relays i and 16. The latter is accomplished by discharging condenser 28 through resistance 21, back contact of relay 23 and the front of relay [5. Relays I6 and i6 will then remain operated for a time after the release of relay l5 while condenser 28 is being charged up through their windings and the resistance shown. This prevents the return of any delayed echoes due to room reverberations.

Speech energy from the lower half of transformer 31 is applied through the loss network 59. and H to rectifier 61, and if of suiiicient magnitude, operates the master guard relay 20.

' This in turn operates the guard relay 2| which is given hangover time by the usual arrangementof resistances and condenser 29 indicated. As long as either one of these relays is operated,

it will be impossible for speech waves to operate the break relay 22. Thus, when receiving speech is sufliciently strong to cause very strong echoes, operation of the guard relays prevents operation of the break relays by echoes. As noted previously, this is necessary to prevent strong echoes from breaking in and possibly causing singing.

At the same time these speech waves are passed through circuit 52 to operate the two expanders, thus removing loss and allowing speech to pass through to the loud-speakers LS3 and LS4.

It is now apparent that if the east talker, for example, is speaking in a normal tone of voice, he will operate the suppressor relays i6 and IS a large portion of the time and will also remove practically all of the loss from the expanders. Due to their substantially lower sensitivity, he will only occasionally operate the guard relays and 2|. Thus if the west talker desires to break in, he may do so by raising his voice sufiiciently to operate the break relays 22 and 23, provided that he speaks for a suiiicient time to operate these'relays in intervals in which the guard relays are not'operated. As noted previously, the difference in sensitivity of the guard relays and suppressor relays causes the guard relays to be operated a much smaller part of the time than the suppressor relays and the result is that break-in is, as a rule, quite easy on this system.

From the foregoing description it will be seen that the loss in each path may be changed in the same manner as shown in the aforesaid patent to Mitchell, 2,267,622, but in such a way as to keep the loss in both paths always the same. In that way the proper relationship for stereophonic transmission is always maintained, and in addition singing or echo difliculties, which might result if the losses in the two paths were difierent, are avoided.

While this invention has been disclosed as embodied in a particular form of an arrangement of parts, it is capable of embodiment in other and diflerent forms without departing from the spirit and scope of the appended claims.

What is claimed is:

1. A stereophonic speech transmission system comprising two one-way transmitting circuits,

each of the said circuits having a transmitter at one and the same end thereof, and having a loud-speaker at the other end thereof, volumelimiting devices in each of said circuits, and

means controlling said volume-limiting devices by the resultant of the currents in both circuits, to reduce equally the volume in each circuit, thereby maintaining the proper relationship for stereophonic speech transmission.

2. A stereophonic speech transmission system comprising two one-way transmitting circuits, two one-way receiving circuits, means connected to each transmitting, circuit to regulate the transmission characteristics of that circuit, and

means connected to both receiving circuits and both incoming paths to control the loss-introducof the speech currents in both incoming paths to render inoperative the said breaking means whenever the intensity of the current in the incoming paths reaches a predetermined level.

5. A two-way stereophonic speech transmission system comprising at each station two outgoing speech paths, and also at each station two incoming speech paths, each of the said outgoing paths at each station having a transmitter connected thereto and each of the said incoming paths at each station having a loudspeaker con nected thereto whereby speech may be stereophonically transmitted and reproduced, means in the outgoing paths at each station to introduce a loss therein to prevent singing when the sound produced by the loud-speakers at a given station reaches a predetermined level, and means controlling the said loss-introducing means by the resultant of the currents in both incoming paths at that station.

6. The system defined by claim 5 further characterized by a breaking means controlled by the resultant of the currents in both outgoing paths from a given station to remove the loss put in the outgoing paths by the loss introducing means in the said outgoing paths.

7. The system defined by claim 5 further characterized by a breaking means controlled by the resultant of the currents in both outgoing paths from a given station to remove-the loss put in aasaeeo the outgoing paths by the loss introducing'means in the said outgoing paths, and means controlled by the resultant of the currents in the incoming paths, whenever that resultant reaches a predetermined level to render inefiective the said breaking means, thereby leaving the loss-introducing means in the outgoing paths under the control of the speech currents in the incoming paths.

8. In a two-way stereophonic speech transmission system, the combination with two circuits both arranged to transmit from east to west, of two other circuits both arranged to transmit from west to east, each of said circuits having a transmitter at one end and a loud-speaker at the other end thereof, a volume limiter in each circuit, means for controlling the pair of volume limiters in the westbound circuits by the joint efiect of the currents in both of the latter circuits, similar means in the eastbound circuits similarly controlled, means in the westbound circuits' to introduce a loss therein, means connected to both eastbound circuits and responsive to the joint efieot of the currents therein to control the loss-introducing means in the westbound cir cuits, and similar loss-introducing means in the eastbound circuits controlled by the joint efiect of the currents in the westbound circuits.

DOREN MITCHELL. 

